When cells are cultivated in a vessel such as a microplate, dish or culturing bottle, some of living cells die if a concentration of the living cells becomes so high as to reach a so-called "full growth" state. To prevent this, it is necessary to change a culture medium and/or to subculture the cells before the full growth state is reached. Hitherto, a technician determines a timing when the cell culture solution is changed or the subculturing is started with his skill by observing the cell proliferation of the cells in the vessel through a microscope. However, such conventional method is not quantitative and therefore not accurate in determining the cell proliferation of cultured cells and the concentration of living cells. In addition, it is time-consuming and has difficulty in precisely observing the cell proliferation of a large amount of cultured cells.
Further, when cells are cloned by a limiting dilution-culture method or when cells are fractionated, both the number of living cells and that of dead cells should be simultaneously measured. In such cases, even if a blood cell counter is used, visual measurement with the microscope includes large error and takes a lot of time.
A cell detection method using image processing has been proposed instead of visual observation with the microscope. However, it has low detection efficiency since it is done simply by binary digitizing. Although detection of the cells is easy when a background is simple, for example, in case of an agar medium on which colonies are formed, a pattern of the cells and the background are both complicated since the cells in the culturing vessel contain dead cells, dust or other foreign substances.